ARRAY INSTRUMENTATION

The array instruments consisted of one-component and three-component sensors, and the data recording units were Reftek Data Acquisition Systems (DAS's) with 16-bit and 32-bit digitizers (Table 2) which ran Reftek CPU Versions 2.45, 2.53, 2.53H, and 2.53J. The DASís recorded either 25 or 50 sps, depending on data storage space limitations. They were supplied by the Incorporated Research Institutes for Seismology - Program for Array Seismic Studies of the Crust and Lithosphere (IRIS/PASSCAL). The sensor types included Mark Products L4C3D 1Hz (3-component), L4C1D 2Hz (1-component), L4C3D 2Hz (3-component), and L22 2Hz (3-component), and the data were collected by exchanging recorders with internal disks or by copying internal disks to tape (see Tables 2 and 3 for instrument configurations). Each instrument recorded data in record lengths of 30 minutes and each 30-minute interval was automatically written to the internal or external disks. The sensors were borrowed from the collections of UCLA, Los Alamos National Lab, the USGS, the IRIS/PASSCAL instrument facility at the Lamont-Doherty Earth Observatory, the Southern California Earthquake Center (SCEC), and USC.

DASís with external hard disks were either swapped each week or the data were copied to tapes in the field using field Exabyte and DAT tape drives. All instruments were powered by car batteries (provided by the USGS, IRIS/PASSCAL, and Princeton University) which had to be changed each week. The sensors were interspersed so that no segment of the array would have only one type; however, all stations near the San Andreas fault were three-component.

Every station was visited approximately once a week; the visit times correspond to the beginning time of DAS recording for each week given in Table 3. Events related to timing receivers, power, temperature, and data acquisition were automatically recorded in the log files for each DAS. Operators manually recorded the information requested on the forms shown in Fig. 2. Appendix A contains a summary of operator notes taken during the station visits which recorded any unusual occurrences that may be relevant to data analysis, not given by the log files (which were stored together with the data).

The timing of each instrument was controlled by GPS receivers (Reftek Version 2.2), Omega receivers, or by the Reftek internal clocks. GPS receivers were connected at each visit to calibrate the internal Reftek clock for some stations. Others had continuously connected GPS or Omega receivers (Table 3). A few stations did not receive external time calibration pulses or hookups and may not be reliable in timing as a result. Stations calibrated with Omega receivers contain an uncorrected time error of exactly 1 s due to incorrect leap second firmware input. In addition, a few stations have a 10-second error due to operator input of incorrect time pulses. The log files stored with the data contain detailed information about the type of timing device used with each instrument and the exact times at which calibrations were performed. Corrections made for other timing errors will be described in the section on data processing. The timing receivers were borrowed from IRIS/PASSCAL at Lamont.

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